The present invention generally relates to the preparation of semiconductor grade single crystal silicon which is used in the manufacture of electronic components. More particularly, the present invention relates to single crystal silicon ingots and wafers, as well as a process for the preparation thereof, having (i) an outer, axially symmetric region wherein silicon interstitials are the predominant intrinsic point defect and which is devoid of agglomerated intrinsic point defects, surrounding (ii) an inner, axially symmetric region wherein silicon lattice vacancies are the predominant intrinsic point defect and which is substantially free of nuclei which lead to the formation of oxidation induced stacking faults.
Single crystal silicon, which is the starting material in most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by, for example, decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
It is now recognized that a number of defects in single crystal silicon form in the growth chamber as the ingot cools from the temperature of solidification. More specifically, as the ingot cools intrinsic point defects, such as crystal lattice vacancies or silicon self-interstitials, remain soluble in the silicon lattice until some threshold temperature is reached, below which the given concentration of intrinsic point defects becomes critically supersaturated. Upon cooling to below this threshold temperature, a reaction or agglomeration event occurs, resulting in the formation of agglomerated intrinsic point defects.
As has been reported elsewhere (see, e.g., U.S. Pat. Nos. 5,919,302 and 6,254,672, as well as PCT/US98/07365 and PCT/US98/07304, all of which are incorporated in their entirety herein by reference), the type and initial concentration of these point defects in the silicon are determined as the ingot cools from the temperature of solidification (i.e., about 1410° C.) to a temperature greater than about 1300° C. (i.e., about 1325° C., 1350° C. or more); that is, the type and initial concentration of these defects are controlled by the ratio v/G0, where v is the growth velocity and G0 is the average axial temperature gradient over this temperature range. Specifically, for increasing values of v/G0, a transition from decreasingly self-interstitial dominated growth to increasingly vacancy dominated growth occurs near a critical value of v/G0 which, based upon currently available information, appears to be about 2.1×10−5 cm2/sK, where G0 is determined under conditions in which the axial temperature gradient is constant within the temperature range defined above. Accordingly, process conditions, such as growth rate (which affect v), as well as hot zone configurations (which affect G0), can be controlled to determine whether the intrinsic point defects within the single crystal silicon will be predominantly vacancies (where v/G0 is generally greater than the critical value) or self-interstitials (where v/G0 is generally less than the critical value).
Defects associated with the agglomeration of crystal lattice vacancies, or vacancy intrinsic point defects, include such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, and crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques (such as Scanning Infrared Microscopy and Laser Scanning Tomography).
Also present in regions of excess vacancies, or regions where some concentration of free vacancies are present but where agglomeration has not occurred, are defects which act as the nuclei for the formation of oxidation induced stacking faults (OISF). It is speculated that this particular defect, generally formed proximate the boundary between interstitial and vacancy dominated material, is a high temperature nucleated oxygen precipitate catalyzed by the presence of excess vacancies; that is, it is speculated that this defect results from an interaction between oxygen and “free” vacancies in a region near the V/I boundary.
Defects relating to self-interstitials are less well studied. They are generally regarded as being low densities of interstitial-type dislocation loops or networks. Such defects are not responsible for gate oxide integrity failures, an important wafer performance criterion, but they are widely recognized to be the cause of other types of device failures usually associated with current leakage problems.
The density of such vacancy and self-interstitial agglomerated defects in Czochralski silicon is conventionally within the range of about 1×103/cm3 to about 1×107/cm3. While these values are relatively low, agglomerated intrinsic point defects are of rapidly increasing importance to device manufacturers and, in fact, are now seen as yield-limiting factors in device fabrication processes.
Agglomerated defect formation generally occurs in two steps; first, defect “nucleation” occurs, which is the result of the intrinsic point defects being supersaturated at a given temperature. Once this “nucleation threshold” temperature is reached, intrinsic point defects agglomerate. The intrinsic point defects will continue to diffuse through the silicon lattice as long as the temperature of the portion of the ingot in which they are present remains above a second threshold temperature (i.e., a “diffusivity threshold”), below which intrinsic point defects are no longer mobile within commercially practical periods of time. While the ingot remains above this temperature, vacancy or interstitial intrinsic point defects diffuse through the crystal lattice to sites where agglomerated vacancy defects or interstitial defects, respectively, are already present, causing a given agglomerated defect to grow in size. Growth occurs because these agglomerated defect sites essentially act as “sinks,” attracting and collecting intrinsic point defects because of the more favorable energy state of the agglomeration.
Accordingly, the formation and size of agglomerated defects are dependent upon the growth conditions, including v/G0 (which impacts the initial concentration of such point defects), as well as the cooling rate or residence time of the main body of the ingot over the range of temperatures bound by the “nucleation threshold” at the upper end and the “diffusivity threshold” (which impacts the size and density of such defects) at the lower end. As has been previously reported (see, e.g., U.S. Pat. No. 6,312,516 and PCT Patent Application Serial No. PCT/US99/14287, both of which are incorporated in their entirety herein by reference), control of the cooling rate or residence time enables the formation of agglomerated intrinsic point defects to be suppressed over much larger ranges of values for v/G0; that is, controlled cooling allows for a much larger “window” of acceptable v/G0 values to be employed while still enabling the growth of substantially defect-free silicon.
It is to be noted, however, that in addition to the formation of agglomerated intrinsic point defects the formation of oxygen precipitate-related defects, such as oxidation induced stacking faults, are also of concern. More specifically, it is to be noted that in addition to the diffusion of intrinsic point defects, when present oxygen can also diffuse through the crystal lattice. If the oxygen concentration is sufficiently high, the formation of oxygen precipitate nucleation centers, as well as oxygen precipitates, can also occur. Silicon wafers, derived from silicon ingots containing such nucleation centers or precipitates, can be problematic for integrated circuit manufactures, because they can lead to oxygen-related defects, such as oxidation induced stacking faults, upon exposure to the thermal conditions of a manufacturing process.
Accordingly, it is desirable to have a single crystal silicon growth process which enables both the control of agglomerated intrinsic point defects as well as the control of oxygen precipitate nucleation centers or oxygen precipitates, particularly those leading to the formation of oxidation induced stacking faults. Such a process would be especially beneficial when growth of medium to high oxygen content silicon (e.g., about 14 to 18 PPMA oxygen content), is needed.